This invention relates to a method of plasma spraying of polymer compositions onto a substrate or other surface, and in particular to such a method in which selected parameters of the plasma spraying process are controlled so as to melt the polymer composition and deliver it to the substrate or other surface before either re-fusing or chain scissioning of the composition. The invention also relates to a new use and operation of an existing plasma spray system for spraying polymers onto a target surface to provide a well-consolidated, dense and void-free coating for the surface.
Devices for generating plasma streams or jets have been used for some time to spray heat fusible materials, primarily metals, ceramics, cermets (combinations of metals and ceramics), and carbides onto a substrate or work piece. See, for example, U.S. Pat. Nos. 5,047,612, 4,970,364, 4,866,240, 4,741,286, 4,505,945, 4,670,290, 4,896,017 and 5,013,883, and the references cited therein. Such devices typically include a cathode and a combination anode and nozzle for generating an electric arc in an ionizable gas upon application of an electric current to the cathode and anode. The ionizable gas, referred to as "plasma gas" hereafter, is passed under pressure into the space between the cathode and anode as a result of which electrons are stripped from the gas to ionize it and thus produce a high energy, high temperature plasma jet which issues from the anode/nozzle. Heat fusible particulate materials are introduced into the plasma jet generally by an inert carrier gas, where the particles are softened or melted while also being accelerated to high velocities. The softened or melted particles are then projected or sprayed onto a substrate or work piece to coat the substrate or work piece. Such coatings serve a variety of purposes including wear resistance, electrical or heat insulation, electrical conduction, friction reduction, corrosion resistance, etc.
Because the particulate materials sprayed with the above-described plasma spray devices have relatively high melting points, the devices are operated to generate especially high temperature plasma jets so that the materials, when introduced into the plasma jet, will melt or nearly melt and remain in that state until they strike the target surface. Because of this, the few attempts to utilize these prior art devices for plasma spraying of polymers, which have relatively low melting points, have met with limited success. Any resulting coatings are degraded, uneven, full of voids, and generally inadequate for the purposes for which the coatings are applied. However, because of the many desirable properties of polymers and polymer coatings (based on experience applying the coatings using other techniques), there is great interest in finding an economical and consistently reproducible solution to the polymer vaporization and degradation problem in plasma spraying of polymers.
Two approaches to solving the above-described problem have been suggested, one in U.S. Pat. No. 4,694,990, and the other in U.S. Pat. No. 5,041,713. In the first mentioned patent, the approach suggested is to include a baffle or flame barrier between the source of the plasma jet and the location of introduction of the particulate material into the plasma jet, to thereby attempt to reduce the combustion or oxidation of the material. However, use of such a barrier would appear to reduce plasma jet velocity and so reduce the rate at which polymers can e applied to a substrate. Further, with the reduction of plasma jet velocity, the coatings applied do not adhere as well to the substrate since bond strength is, in part, dependent upon speed of impact of the coating material. Finally, provision of such a barrier increases the cost and decreases the reliability of the plasma spray system, and also decreases the precision of the spraying process as to uniformity and thickness of the coatings.
In U.S. Pat. No. 5,041,713, the approach taken to avoiding overheating of the polymers is to vary the location downstream of the nozzle at which the polymers are injected into the plasma jet while also attempting to cool the plasma jet by use of a fluid-cooled nozzle which employs both a cooling liquid and a cooling gas. It has been found, however, that simply cooling of the nozzle and varying the location of injection of the polymers into the plasma jet is generally insufficient to ensure that the polymers both melt and are delivered to the target surface before re-fusing, chain scissioning, combusting or vaporizing. Generally, the approach in the '713 patent lacks the versatility to allow for the successful plasma spraying of a wide variety of polymers.
Although not mentioned in the above two referenced patents, others have suggested that for plasma flame spraying of metals and ceramics, control of some parameters of the plasma spraying process would serve to allow for softening the metal and/or ceramic particles without melting the particles, and then for delivering the particles to a surface to be coated. In particular, U.S. Pat. No. 3,914,573 discloses the process of controlling the dwell time of particles in a plasma stream by controlling the angle and position of injection of the particles into the stream at the throat of the nozzle of the plasma spray gun. The "throat" is defined in the patent as that area within the nozzle between the source of the plasma stream and the exit point of the nozzle. The angle and position of injection of the particles is changed by changing nozzles having different angles and positions for the injection conduit. For plasma spraying of metals and ceramics, such an approach to controlling the softening of the metal and ceramic particles may be acceptable, but it would not be acceptable nor even workable for plasma spraying of polymers because their melting points are generally lower than those of metallics and ceramics. Injection at the location indicated in the nozzle throat of the '573 patent would likely result in the polymers being vaporized or at least thermally degraded so that any resulting coating would not serve the purposes desired. This is because the temperature of the plasma stream in the nozzle throat would be too high.
The plasma stream exit velocity in the '573 patent, suggested at between Mach 1 and Mach 3, with Mach 2 being preferred, would seemingly assist in rapidly delivering the coating material to the substrate before vaporization could take place. However, with polymers, such velocities would still not likely be sufficient to prevent vaporization o thermal degradation before the polymers could reach and coat the substrate. In other words, the approach recommended by the '573 patent for plasma spraying of metals and ceramics would generally not work for spraying polymers.
In general, the prior art, although acknowledging the desirability of plasma spraying of polymers to coat substrates and other surfaces, fails to appreciate the critical parameters in the plasma spraying process which will yield polymer coatings having the desired characteristics. It is important that the polymers, after melting, be maintained in the melt state so as not to either re-fuse (solidify) or chain scission (breaching or breaking of the chain structure of the polymer) before striking the surface to be coated. Polymers can be damaged or degraded at a temperature short of combustion or vaporization. Such damage (chain scissioning) can occur from a combination of too much heat over too long a period of time (the greater the heat, the less time is required, and vice versa). The prior art has failed to recognize that these critical events and phases in the plasma spraying of polymers may be achieved and maintained by carefully controlling selected parameters of the plasma spraying process to thereby effectively and efficiently plasma spray a variety of polymer coatings onto target surfaces.